The present invention relates to an instrument which measures a time interval of an input signal thereto, that is, a time signal of a pulse interval or width corresponding to the time interval to be measured, and more particularly to a time interval measuring instrument which measures the durations of input time signals within a predetermined range.
For example, in a digital audio player system, it is customary, for evaluating its quality, to measure fluctuations in the pulse width of pulses available from a pickup, what is called jitter. In the digital audio player, audio information is rendered into a PCM (Pulse Code Modulation) code and its PCM code pulse train is recorded as an NRZ (Non-Return to Zero) waveform. On account of this, the output from the pickup includes pulses having various widths at random, which are an integral multiple of a fundamental clock period. For measuring jitter of the pulse width, it is necessary to extract a pulse of a nominal predetermined pulse width. Not only in the digital audio player but also in apparatuses which usually employ a PCM code train or a so-called data pulse train, as an NRZ waveform, for various data transfers, data storage and data processing, it is necessary, for measuring jitter of the pulse width, to conduct measurements for pulses of a specified nominal width among a number of pulses of various widths which exist at random.
In the case of measuring jitter of pulses of various pulse widths by the existing time interval measuring instrument, such pulse signals (time signals) as, for example, those from the pickup of the aforesaid digital audio player, are input into the time interval measuring instrument, wherein the input pulse signals are extracted at random and their pulse widths (durations) measured, and the measured data are transferred to a controller. The controller is constituted by a microcomputer, in which only data on a specified pulse width are extracted from the measured data transferred thereto and the amount of jitter of the pulse is statistically obtained from the extracted data. For example, a mean value of the extracted measured values, a standard deviation of the amount of jitter and a difference between minimum and maximum values of the pulse widths (what is called a range) are calculated.
For obtaining each statistics as the mean value and the standard deviation, at least 100 measured data are desirably needed. However, the abovementioned conventional measuring instrument requires, for each measurement, a total of T.sub.X +T.sub.R +T.sub.C which is the sum of a time T.sub.X for measuring the time interval (the pulse width), a time T.sub.R for transferring the measured data to the controller and a time T.sub.C for deciding whether the measured data corresponds to the specified nominal pulse width. The data transfer is carried out by, for instance, the GP-IB transfer procedure, but the data transfer time T.sub.R is as long as 10 milliseconds or so. An increase in the time for each measurement inevitably causes an increase in the time necessary for obtaining a required number of measured data. For example, if the time for each measurement is about 100 milliseconds, then 10 seconds or so is needed for obtaining 100 measured data. Moreover, it must be taken into account that all the 100 measured data thus obtained in succession need not necessarily be those of the specified pulse width but contain many inappropriate data of other pulse widths than the specified one. Accordingly, it is necessary, in practice, to obtain more measured data and to extract therefrom appropriate data of the specified pulse width. Therefore, for instance, if 1,000 to 10,000 measured data are needed for obtaining 100 measured data of the specified nominal pulse width, then the measurement time may require 100 to 1000 seconds, resulting in much time being consumed for the measurement of jitter.